Submerged water desalination system with replaceable dockable membrane modules

ABSTRACT

A submersible water desalination apparatus includes an array of hot-swappable water separation membrane modules arrayed in an approximately polygonal configuration around a product water collection conduit that represents or is proximate to and generally aligned with a central axis of the array; a plurality of hot-swap product water valves connected to the conduit and detachably connected to generally radially-extending product water collection manifolds in fluid communication with a plurality of water separation membrane cartridges within the modules; the modules having in cross-section generally tapered module sides that converge towards the conduit and assist in underwater docking and attachment of a replacement module to a hot-swap product water valve.

The present application is a continuation under 35 U.S.C. § 111(a) ofInternational Patent Application No. PCT/US2020/058569, filed on Nov. 2,2020, which claims priority to U.S. Provisional Patent Application No.62/929,564, filed on Nov. 1, 2019, the disclosures of which areincorporated by reference herein.

TECHNICAL FIELD

This invention relates to water desalination.

BACKGROUND ART

The growth of saltwater (e.g., seawater) desalination has been limitedby the relatively high cost of desalinated water. This high cost is duein part to energy and capital expenses associated with currentdesalination systems. Such systems typically employ an onshore facilitycontaining reverse osmosis (RO) desalination membranes contained inhigh-pressure corrosion-resistant housings and supplied with seawaterfrom a submerged offshore intake system. Very high pressures typicallyare required to drive water through the RO membranes. For example, thewidely-used FILMTEC™ SW30 family of reverse osmosis membrane elements(from DuPont Water Solutions) require about an 800 psi (55 bar) pressuredifferential across the membrane to meet design requirements. Inaddition to such high pressures, onshore RO units suffer from high powerdemands, primarily for pressurizing the feedwater to membrane operatingpressures, and for an onshore RO unit these power demands typicallyaverage about 13.5 kWh per thousand gallons of produced fresh water. Theseawater and the concentrated brine stream produced by a typical onshoreRO unit have high corrosion potential and consequently require expensivecomponents and equipment, including pressure vessels and conduits madefrom specialized alloys. The highly-pressurized water flow alsoincreases maintenance expenses. Onshore RO units typically also requiresignificant amounts of expensive seaside real estate. Shore-baseddesalination has in addition been criticized for various environmentalimpacts, including entrainment of marine life in the intake water,greenhouse gas production associated with producing the energy required,discharge of a strong brine stream with the potential to harm marinelife, the use of treatment chemicals that may enter the ocean, andonshore land use that is often expensive and may harm local ecosystems.RO units include those described in U.S. Pat. No. 4,334,992 (Bonin etal.), U.S. Pat. No. 5,192,434 (Moller), U.S. Pat. No. 5,620,605 (Molleret al.), U.S. Pat. No. 5,788,858 (Acernase et al. '858), U.S. Pat. No.5,972,216 (Acernase et al. '216), U.S. Pat. No. 8,282,823 B2 (Acernaseet al. '823) and U.S. Pat. No. 9,227,159 B2 (DuFresne et al.).

In the 50 years since the invention of semi-permeable RO membranes,various concepts for submerging such membranes and employing naturalhydrostatic water pressure to help desalinate seawater have beenproposed. Representative examples include the systems shown in U.S. Pat.No. 3,171,808 (Todd), U.S. Pat. No. 3,456,802 (Cole), U.S. Pat. No.4,125,463 (Chenowith), U.S. Pat. No. 5,229,005 (Fok et al.), U.S. Pat.No. 5,366,635 (Watkins), U.S. Pat. No. 5,914,041 (Chancellor '041), U.S.Pat. No. 5,944,999 (Chancellor '999), U.S. Pat. No. 5,980,751(Chancellor '751), U.S. Pat. No. 6,149,393 (Chancellor '393), U.S. Pat.No. 6,348,148 B1 (Bosley) and U.S. Pat. No. 8,685,252 B2 (Vuong et al.);US Patent Application Publication Nos. US 2008/0190849 A1 (Vuong), US2010/0270236 A1 (Scialdone) US 2010/0276369 A1 (Haag) and US2018/0001263 A1 (Johnson et al.); GB Patent No. 2 068 774 A (Mesple);International Application Nos. WO 00/41971 A1 (Gu), WO 2009/086587 A1(Haag Family Trust), WO 2018/148528 A1 (Bergstrom et al.), WO2018/148542 A1 (Bergstrom) and Pacenti et al., Submarine seawaterreverse osmosis desalination system, Desalination 126, pp. 213-18(November, 1999).

Other water desalination technologies have also been proposed, includingsystems employing microfiltration, nanofiltration, ultrafiltration andaquaporins. These likewise have various drawbacks. In general, submergedwater desalination systems do not appear to have been placed inwidespread use, due in part to factors such as the energy cost ofpumping the desalinated water to the surface from great depth and thedifficulty of maintaining component parts at depth.

From the foregoing, it will be appreciated that what remains needed inthe art is an improved system for water desalination featuring one ormore of lower energy cost, lower capital cost, lower operating ormaintenance cost or reduced environmental impact. Such systems aredisclosed and claimed herein.

SUMMARY

Compared to land-based water desalination, a submerged waterdesalination system can provide several important advantages. Forexample, submerged operation can significantly reduce pump powerrequirements, since hydrostatic pressure can provide much or all of thedriving force required for desalination, and only desalinated water willneed to be pumped onshore. Eventually however, whether operated onshoreor submerged, water separation membranes may become fouled by scaling ordebris or may otherwise lose their effectiveness, and will need to bereplaced. Replacement can be difficult, especially when the membranesare submerged at significant depths, and may require shutting down anentire submerged desalination apparatus or in some cases bringing it tothe surface so that membrane or cartridge replacement can be carriedout. Although some submerged RO (SRO) systems have been said to allowmembrane or cartridge replacement while the SRO apparatus is submerged,replacement remains difficult. This can be especially important when thesubmerged apparatus is located at a depth sufficient to require use of aremotely operated vehicle (ROV) for servicing, or when located in aregion with moving subsea currents, such as certain tidal areas, narrowstraits or some submarine canyons.

The disclosed invention provides in one aspect a submersible waterdesalination apparatus comprising an array of hot-swappable waterseparation membrane modules arrayed around and generally radiallyextending from a product water collection conduit; a plurality ofhot-swap product water valves connected to the conduit and detachablyconnected to generally radially-extending product water collectionmanifolds in fluid communication with a plurality of water separationmembrane cartridges within the modules; the modules having incross-section generally tapered module sides that converge towards theconduit and assist in underwater docking and attachment of a replacementmodule to a hot-swap product water valve.

The disclosed invention provides in another aspect a method formaintaining a submerged water desalination apparatus, the methodcomprising the steps of:

-   -   i) undocking from a submerged water desalination apparatus a        first hot-swappable water separation membrane module that is a        member of an array of such modules that are arrayed around and        generally radially extend from a product water collection        conduit and are in fluid communication with the conduit via        hot-swap product water valves connected to the conduit and        detachably connected to generally radially-extending product        water collection manifolds in fluid communication with a        plurality of water separation membrane cartridges within the        modules, the modules having in cross-section generally tapered        module sides that converge towards the conduit and assist in        underwater docking and reattachment of detached modules to a        hot-swap product water valve; and    -   ii) docking a similarly sized and shaped replacement module and        attaching it to the array while underwater by urging the        converging tapered sides of the replacement module towards an        available hot-swap product water valve.

The disclosed apparatus provides a submerged “Natural Ocean Well” thatcan provide desalinated water at reduced cost and with improvedreliability compared to land-based RO systems, and with improved ROmembrane maintenance and replacement compared to existing SRO systems,especially when replacement is accomplished using an ROV

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 and FIG. 2 are schematic side views of one embodiment of thedisclosed apparatus;

FIG. 3 and FIG. 4 are respectively side and schematic views of OceanThermal Energy Conversion (“OTEC”) systems for supplying power to thedisclosed apparatus;

FIG. 5 is a side view of alternate systems for supplying power to thedisclosed apparatus, and for disposing of concentrate or brine withreduced greenhouse gas emissions;

FIG. 6 is a perspective view of a water farm formed by a connected arrayof the disclosed water desalination systems;

FIG. 7A, FIG. 7B and FIG. 7C are three perspective underside views ofthe disclosed apparatus with and without certain components;

FIGS. 8A through 8E are perspective topside or underside views, fromseveral observation angles, of an individual hot-swappable waterseparation membrane module;

FIGS. 8F and 8G are cross-sectional views showing the use of an adhesiveto bond and seal water separation cartridges in a water separationmembrane module;

FIG. 9 is a top plan view of a generally polygonal array of thedisclosed modules showing a module in an unattached condition;

FIG. 10A through FIG. 10C are perspective views of the disclosed modulesarranged for shipment on a flatbed trailer; and

FIG. 11 and FIG. 12 are outlet and inlet side schematic views of aprefilter cleaning system.

Like reference symbols in the various figures of the drawing indicatelike elements. The elements in the drawing are not to scale.

DETAILED DESCRIPTION

The recitation of a numerical range using endpoints includes all numberssubsumed within that range (e.g., 1 to 5 includes 1, 1.5, 2, 2.75, 3,3.80, 4, 5, etc.).

The terms “a,” “an,” “the,” “at least one,” and “one or more” are usedinterchangeably. Thus, for example, an apparatus that contains “a”reverse osmosis membrane includes “one or more” such membranes.

The term “airlift” when used with respect to a pump refers to a deviceor method for pumping a liquid or slurry by injecting air (andpreferably only by injecting air) into the liquid or slurry.

The term “automatic” when used with respect to control of a submergedpump means that the control operates in the vicinity of and based onconditions in such pump, and without requiring the sending of signals toor the receipt of signals from a surface vessel, platform, or othernon-submerged equipment.

The term “brine” refers to an aqueous solution containing a materiallygreater sodium chloride concentration than that found in typicalsaltwater, viz., salinity corresponding to greater than about 3.5%sodium chloride. It should be noted that different jurisdictions mayapply differing definitions for the term “brine” or may set differentlimitations on saline discharges. For example, under current Californiaregulations, discharges should not exceed a daily maximum of 2.0 partsper thousand (ppt) above natural background salinity measured no furtherthan 100 meters horizontally from the discharge point. In otherjurisdictions, salinity limits may for example be set at levels such as1 ppt above ambient, 5% above ambient, or 40 ppt absolute.

The term “concentrate” refers to an RO apparatus discharge stream havingan elevated salinity level compared to ambient surrounding seawater, butnot necessarily containing sufficient salinity to qualify as brine inthe applicable jurisdiction where such stream is produced.

The term “conduit” refers to a pipe or other hollow structure (e.g., abore, channel, duct, hose, line, opening, passage, riser, tube orwellbore) through which a liquid flows during operation of an apparatusemploying such conduit. A conduit may be but need not be circular incross-section, and may for example have other cross-sectional shapesincluding oval or other round or rounded shapes, triangular, square,rectangular or other regular or irregular shapes. A conduit also may bebut need not be linear or uniform along its length, and may for examplehave other shapes including tapered, coiled or branched (e.g., branchesradiating outwardly from a central hub).

The term “depth” when used with respect to a submerged waterdesalination apparatus or a component thereof refers to the verticaldistance, viz., to the height of a water column, from the free surfaceof a body of water in which the apparatus or component is submerged tothe point of seawater introduction into the apparatus or to the locationof the component.

The terms “desalinated water”, “fresh water” and “product water” referto water containing less than 1000 parts per million (ppm), and morepreferably less than 500 ppm, dissolved inorganic salts by weight.Exemplary such salts include sodium chloride, magnesium sulfate,potassium nitrate, and sodium bicarbonate.

The term “recovery ratio” when used with respect to an SRO system or SROapparatus means the volumetric ratio of product water (permeate)produced by the system or apparatus to feedwater introduced to thesystem or apparatus.

The terms “remotely operated vehicle” and “ROV” refer to unoccupiedsubmersible vehicles capable of underwater maneuvering and manipulationof submerged objects.

The terms “saltwater” and “seawater” refer to water containing more than0.5 ppt dissolved inorganic salts by weight, and thus encompassing bothbrackish water (water containing 0.5 to 3.0 ppt dissolved organic saltsby weight) as well as water containing more than 3.0 ppt dissolvedorganic salts by weight. In oceans, dissolved inorganic salts typicallyare measured based on Total Dissolved Solids (TDS), and typicallyaverage about 35 ppt TDS, though local conditions may result in higheror lower levels of salinity.

The term “submerged” means underwater.

The term “submersible” means suitable for use and primarily used whilesubmerged.

The term “wide area” when used with respect to dispersal of a fluid(e.g., concentrate or brine) away from a conduit having a plurality offluid outlets (e.g., concentrate or brine outlets) distributed along alength of the conduit, means dispersal into an outfall area, andtypically into a volume, encompassing at least 5 meters of such length.The disclosed area or volume will also have other dimensions (e.g., awidth, diameter or height) that will depend in part upon the directionand velocities of fluid streams passing through the fluid outlets.Because such other dimensions will be affected by variable factorsincluding fluid flow rates inside and outside the conduit, and theoverall shape of the dispersed fluid plume, the term “wide area” hasbeen defined by focusing merely on the recited length along the recitedconduit, as such length typically will represent a fixed quantity in agiven dispersal system.

In the discussion that follows, emphasis will be placed on the use of ROmembranes in a submerged RO (SRO) apparatus for carrying out waterseparation, it being understood that persons having ordinary skill inthe art of desalination will after reading this disclosure be able toreplace the disclosed RO membranes with other types of water separationmembranes. Exemplary such other water separation membranes include thosebased on microfiltration, nanofiltration and ultrafiltration;aquaporins; and other water separation technologies that are now knownor hereafter developed and which will be familiar to persons havingordinary skill in the desalination art.

Referring first to FIG. 1 and FIG. 2, SRO apparatus 100 is shown inschematic side view. Raw seawater 102 enters apparatus 100 via prefilterscreens 104, and is separated by RO membrane modules 106 into productwater permeate stream 108 and concentrate or brine discharge stream 110.Permeate stream 108 passes into permeate collector 112 and thencethrough permeate conduit 113, submerged pump 114 and delivery conduit116 to a ship-borne or onshore collection point (not shown in FIG. 1 orFIG. 2) for post-treatment, conveyance or storage for later use. Suchuses may include municipal, private or industrial purposes includingpotable water, bathing water, irrigation water, process water, waterstorage, water table replenishment, cooling or heat exchange, and avariety of other purposes that will be apparent to persons havingordinary skill in the desalination art. For example, potential coolingor heat exchange applications for such product water include providingor improving the efficiency of air conditioning systems including SeaWater Air Conditioning (SWAC) systems; operating or improving theefficiency of OTEC systems (in addition to those discussed herein); andoperating or improving the efficiency of Rankine Cycle heat engines(again, in addition to those discussed herein).

In the disclosed apparatus, raw seawater, product water and concentrateor brine may each flow in a variety of directions, e.g., upwardly,downwardly, horizontally, obliquely or any combination thereof. In theembodiment shown in FIG. 2, reverse osmosis membranes within membranemodules 106 are oriented so that concentrate or brine 110 is dischargedgenerally upwardly from the modules 106 and is captured and collected byhood 118. Axial pump 120 located at the lower end of riser 122 sendscaptured concentrate or brine 110 through riser 122 toward surface 124,for further use or dispersal.

In the embodiment shown in FIG. 2, concentrate or brine 110 exits riser122, whereupon dispersion and dilution takes place in the surroundingseawater. In an additional embodiment (not shown in FIG. 2), concentrateor brine 110 is transported through a further conduit to undergodispersal (and preferably wide area dispersal) at a significant distance(e.g., at least 50, at least 100, at least 200, at least 300, at least400 or at least 500 meters) away from apparatus 100, or into a sustainedunderwater current 130, to be swept away from apparatus 100. In afurther embodiment (also not shown in FIG. 2), concentrate or brine 110is transported through a further conduit for an even greater distance(e.g., all the way to or nearly to surface 124) for further use ordispersal. If desired, the concentrate or brine may instead bedischarged in another direction such as downwardly or horizontally,while preferably still undergoing wide area dispersal well away fromapparatus 100.

The concentrate or brine may be used for a variety of purposes prior todischarge. In one embodiment, the concentrate or brine has desirablevolumetric and thermal utility that may be used to operate an OTECsystem and provide operating or surplus power, as discussed in moredetail below and in copending International Application No.PCT/US2020/058567, filed on Nov. 2, 2020 and entitled OCEAN THERMALENERGY CONVERSION SUBMERGED REVERSE OSMOSIS DESALINATION SYSTEM, thedisclosure of which is incorporated herein by reference.

In the embodiment shown in FIG. 2, buoyancy provided by a ring float 126and a foam layer, e.g., an engineered syntactic foam layer (not shown inFIG. 2) located beneath the surface of hood 128, help maintain apparatus100 at an appropriate depth D below surface 124. Catenary mooring lines132 affixed to anchors 134 in seabed 136 help maintain apparatus 100 atan appropriate depth D below surface 124, an appropriate height H aboveseabed 136, and an appropriate height H′ above the inlet to pump 114.Depth D preferably is such that the hydrostatic pressure of seawater atdepth D is sufficient to drive seawater 102 through membrane modules 106and produce product water 108 and concentrate or brine 110 at a desiredoverall volume and recovery ratio without the need for additional pumpsor other measures to pressurize seawater 102 on the inlet side ofmembrane modules 106. The chosen depth D will vary based on severalfactors including the pressure drop across the above-mentioned prefilterscreens 104; the type, dimensions and arrangement of RO cartridgeswithin the membrane modules 106; the type, sizing and operatingconditions of permeate collector 112, permeate conduit 113, productwater pump 114 and product water conduit 116; and the type, sizing andoperating conditions of axial brine pump 120 and concentrate riser 122.For example, if operating the disclosed SRO apparatus usingHYDRANAUTICS™ SWC cylindrical membrane cartridges from NittoHydranautics operated without pumps to pressurize the inlet seawater,then operation at a depth of at least about 350 m together with a pumpto draw product water from the membrane elements is preferred in orderto minimize or eliminate the need for a high pressure vessel surroundingthe membrane elements. In some prior SRO designs, especially those thatrely on a pressure pump to force seawater through the membranes, thickpressure-resistant vessels are employed to contain the high pressuresneeded for membrane separation. In preferred embodiments of the presentSRO desalination apparatus, the prefilter elements and RO membranes willnot require pressure-resistant vessels, as they will already be immersedat a sufficiently high pressure in the fluid to be purified. Desirablythe disclosed SRO apparatus merely maintains a sufficiently low pressureon the membrane product discharge side, and a sufficient inletside-outlet side pressure differential, so as to allow proper membraneoperation without the use of a surrounding pressure-resistant vessel.

Greater depths than those needed for operation without a pressure vessel(e.g., at least about 400, at least about 450, at least about 500, atleast about 550, at least about 600, at least about 650, at least about700, at least about 750, at least about 800, at least about 900 or atleast about 1,000 m) may be employed if desired, with operation at suchgreater depths increasing the pump suction head and inlet pressure, andenabling use of the same model pump as might be employed at lesserdepths. Such lesser depths may for example be at least about 300, atleast about 200 or at least about 100 m, with operation at such lesserdepths typically requiring at least one pump to help push seawaterthrough the RO membranes (or a suitable vacuum assist on the outletside) in order to achieve efficient desalination, and possibly alsorequiring a pressure vessel surrounding and protecting the membraneelements. Near the surface, the hydrostatic pressure of the ocean willneed to be augmented by mechanical pumping to provide the trans-membranepressure differential needed for water separation. Excessive depths thatmay cause or accelerate pump failure should however be avoided. Overallexemplary depths for operation of the disclosed SRO desalination systemare for example from just below the surface (e.g., from about 10 m),from about 100 m, from about 300 m, or from about 500 m, and up to about2,000 m, up to about 1,500 m or up to about 1,000 m. Depending on thechosen pump and membranes, preferred depths are from just below thesurface to as much as 1500 m depth. Near the surface, the hydrostaticpressure of the ocean typically will need to be augmented by mechanicalpumping to provide the trans-membrane pressure differential needed forreverse osmosis.

Depth D may moreover be a fixed depth chosen at the time ofinstallation, or an adjustable depth that may for example be changedfollowing SRO apparatus startup or changed in response to changingconditions (e.g., changing wave, tidal, thermocline or haloclineconditions, changing seawater salinity, sea level rise, or changes inthe operating efficiency of the RO membranes). In a further embodiment,the disclosed SRO apparatus may include a pressure-seeking capability toenable the system to increase or decrease its depth in order to obtaindesired hydrostatic pressures, to optimize or adjust RO operatingconditions or to optimize or adjust product water and concentrate orbrine delivery.

By way of example, if the disclosed apparatus is operated at a depth ofabout 700 m, hydrostatic pressure will provide approximately 68 bar onthe high-pressure side of the semi-permeable RO membrane. When used witha presently preferred backpressure of 13 bar or less on the productdischarge side of the membrane, this will result in a pressuredifferential across the membrane of 55 bar (approximately 800 psi) ormore. In situations of higher- or lower-salinity waters, these depth andpressure values may vary. The inlet pressure will in any event normallybe the ocean hydrostatic pressure at the chosen SRO operating depth.

The preferred depth and pressure values set out above may vary insystems that take advantage of future membrane developments enabling orrequiring lower or higher differential pressures or higher or lowermembrane backpressures. Adjustments to accommodate such developments mayincrease or decrease the preferred operating depth for the disclosed SROapparatus. For many membranes, the pressure on the low-pressure sidetypically will not change appreciably with depth, and consequentlychanging the depth of operation may suffice to adjust the differentialpressure across the membrane and achieve optimal operating conditions.

The heights H (the vertical spacing between the lowest inlets toprefilter screens 104 and seabed 136) and H′ (the vertical spacingbetween the membrane module 106 product water outlets and the inlet topump 114) in FIG. 2 may for example each represent at least about 3, atleast about 5, at least about 10, at least about 20, at least about 40or at least about 50 m. Lesser heights H and H′ may be employed. Forexample, height H may be reduced to near zero or zero, so that theinlets to prefilter screens 104 are near or at the same depth as seabed136. However, doing so typically will increase the turbidity of seawater102 entering modules 106 and the possibility that foreign matter may bedrawn through prefilter screens 104 and into modules 106. Also, heightH′ between prefilter screens 104 and pump 114 may be reduced to nearzero, zero or even less than zero (viz., by housing pump 114 inside thescreened intake system between the prefilter screens 104 and the modules106). In such reduced height H′ embodiments the pump 114 and prefilterscreens 104 preferably will however remain elevated at a sufficientdistance above seabed 136 to avoid turbidity that may be present nearseabed 136.

The depth of the disclosed apparatus 100, height H′ and the diameter ofthe inlet to pump 114 are desirably sized to provide at least the netpositive suction head (NPSH) or greater pressure (viz., the pressurecaused by the height of the standing column of product water 108 inpermeate conduit 113 and permeate collector 112 between membrane modules106 and the inlet side of pump 114) sufficient to avoid inlet sidecavitation upon startup and operation of pump 114. Further detailsregarding such cavitation avoidance during startup and operation may befound in copending International Application No. PCT/US2020/058573,filed on Nov. 2, 2020 and entitled SUBMERGED WATER DESALINATION SYSTEMWITH REMOTE PUMP, the disclosure of which is incorporated herein byreference.

As depicted in FIG. 2, pump 114 rests upon and if desired may be mooredto seabed 136 or to other natural or artificial structures on theseabed. Pump 114 may if desired be suspended above the seabed, forexample in locations where the seabed is uneven or inclined. In oneembodiment, pump 114 is suspended beneath the remainder of apparatus 100by underwater mooring lines affixed to the apparatus and pump. Pump 114may if desired be located in other locations, for example affixed to anoffshore oil or gas platform, offshore wind farm support, bridge pier orother partly or wholly submerged supporting structure.

Pump 114 and the other pumps referred to herein may be selected from awide variety of submersible single stage or multistage pumps, includingpiston (e.g., axial piston), plunger, rotary (e.g., centrifugal impellerpumps and rim-driven shaftless thrusters) and screw pumps that may use avariety of flow schemes including positive displacement, centrifugal andaxial-flow principles. Suitable pumps are available from a variety ofsources that will be familiar to persons having ordinary skill in thedesalination art, and may in appropriate instances be adapted from otherfields such as subsea oil and gas exploration, and marine (includingsubmarine) positioning and propulsion. Exemplary pump suppliers includeBrunvoll, Cat Pumps, Copenhagen Subsea, Enitech, FMC Kongsberg SubseaAS, Fuglesang Subsea AS, Halliburton, Hayward Tyler, Ocean YachtSystems, Parker, Rolls Royce, Schlumberger, Schottel, Silent Dynamics,Technical Supply & Logistics, Vetus and Voith. In some embodiments thedisclosed pumps include hot-swap connectors to enable them to be removedfrom the disclosed apparatus while it is submerged, for replacement,repair or rebuilding.

In some embodiments, pump 114 includes one or more sensors, controls ora torque limiting coupling (e.g., a magnetic clutch, hydraulic torqueconverter, combination thereof or other such device) between theelectrical motor powering the pump and the pump impeller so as to limitor avoid inlet side cavitation and accompanying stress or otherdisturbance of the RO membranes during pump operation. Further detailsregarding cavitation avoidance during such operation may be found incopending International Application No. PCT/US2020/058570, filed on Nov.2, 2020 and entitled SUBMERGED WATER DESALINATION SYSTEM WITH PRODUCTWATER PUMP CAVITATION PROTECTION, the disclosure of which isincorporated herein by reference.

In one embodiment, pump 114 diverts at least a portion of the productwater 108 for use as a lubricating or cooling fluid directed through oneor more of the pump, pump motor or the coupling between the motor andpump. Doing so can improve the pump longevity, while avoiding the needto use seawater, hydraulic fluid or other potentiallyparticulate-bearing, corrosive or toxic fluids for lubrication orcooling. Further details regarding the use of product water for suchlubrication and cooling may be found in copending InternationalApplication No. PCT/US2020/058572, filed on Nov. 2, 2020 and entitledSUBMERGED WATER DESALINATION SYSTEM PUMP LUBRICATED WITH PRODUCT WATER,the disclosure of which is incorporated herein by reference.

Electrical power and appropriate control signals 138 may be supplied topump 114 and other components of apparatus 100 through multi-conductorcable 140. The supplied electrical power operates pumps 114 and 120 andas needed other components in apparatus 100, such as a prefiltercleaning brush system. Further details regarding a desirable prefiltercleaning brush embodiment are discussed in more detail below.

When operated at sufficient depth, the RO membranes in apparatus 100will not need to be encased in pressure vessels, and may instead bemounted in a lightweight supporting frame or other housing made fromrelatively inexpensive and suitably corrosion-resistant materials suchas a corrosion-resistant metal skeleton or a housing made from asuitable plastic, fiber-reinforced (e.g., glass fiber- or carbonfiber-reinforced) plastic or other composite, or a variety of otherunreinforced or engineered plastics the selection of which will beunderstood by persons having ordinary skill in the desalination art.Avoiding the need for a pressure vessel greatly reduces the requiredcapital expenditure (CAPEX) for constructing apparatus 100 compared tothe costs for constructing a shore-based RO unit. If the RO membranesare individual units (for example, cartridges containing spiral-woundmembranes), then avoidance of a pressure vessel also enables modules 106to be economically designed using a parallel array containing asignificantly larger number of cartridges than might normally beemployed in a shore-based RO unit, and operating the individualcartridges at a lower than normal individual throughput. For example,the number of cartridges may be at least 10% more, at least 15% more, atleast 20% more or at least 25% more than might normally be employed inan onshore RO unit. Doing so can help extend the life of individualmembrane cartridges while still providing a desired daily amount ofproduct water. In the embodiment shown in FIG. 1 and FIG. 2, and asdiscussed in more detail below, modules 106 preferably contain a largearray of parallel cylindrical RO cartridges operated not only at suchlow individual throughput, but also with a reduced recovery rate. Doingso can also provide reduced concentrate salinity, reduced foulingpotential, and in preferred embodiments will result in a large volume ofconcentrate that does not qualify as brine in the applicablejurisdiction, and which has substantial cold thermal energy potentialfor cooling an OTEC system. For example, permeate stream 108 is depictedin FIG. 1 as having a substantially smaller volume than brine dischargestream 110, corresponding to a low recovery ratio. Exemplary recoveryratios may for example be no greater than 40%, no greater than 30%, nogreater than 20%, no greater than 15%, no greater than 10%, no greaterthan 8% or no greater than 6%, and may for example be less than 3%, atleast 3%, at least 4% or at least 5%. The chosen recovery ratio willdepend upon factors including the selected RO membranes, and the depthand applicable jurisdiction in which the SRO apparatus operates. Thechosen recovery ratio also influences pump sizing and energy costs. Byway of example, for an SRO embodiment employing Dow FILMTEC membranecartridges to treat seawater with an average 34,000 ppm salinity at an8% recovery ratio, about 8% of the seawater inlet stream will beconverted to product water having less than 500 ppm salinity, and about92% of the seawater inlet stream will be converted to a low pressure orunpressurized brine stream having about 37,000 ppm salinity. By way of afurther example, an SRO apparatus employing Nitto Hydranautics membranecartridges operated at a depth of about 500 m and a 5% recovery ratiomay be used to produce concentrate that does not qualify as brine underthe current version of the California Water Quality Control Plan.

In one preferred embodiment, the disclosed SRO apparatus operates at adepth of at least about 350 m, does not employ seawater pumps on the ROmembrane inlet side, and employs a product (fresh) water pump on theoutlet side of the RO membranes to maintain at least a 27 bar and morepreferably at least a 30 or 35 bar pressure differential across themembranes, to allow the ocean's hydrostatic pressure to force or tolargely help force product water through such membranes. Advantages forsuch a configuration include a pump requiring much less energy whenlocated at the membrane outlet rather than at the inlet, and theavoidance of, or much lower requirements for, any pressure vesselshousing the membranes. Use of membranes with a low required pressuredifferential will enable operation at lesser depths or using smallerpumps. Currently preferred such membranes include Nitto HydranauticsSWC6-LD membranes (40 bar differential pressure) and LG ChemLG-SW-400-ES membranes (38 bar differential pressure).

The pumps in the disclosed apparatus may be supplied with power in avariety of ways. Referring now to FIG. 3, closed-cycle OTEC unit 300employs a recirculating Rankine cycle heat exchange and generator system302 mounted on or within a submerged platform 304 whose location belowsurface level 124 provides desirable protection from wind and waveaction. Platform 304 may be affixed to seabed 136 using catenary mooringlines 306 affixed to anchors 308. Platform 304 may if desired beotherwise fixed in place, for example to an offshore oil or gasplatform, offshore wind farm support, bridge pier or other partly orwholly submerged supporting structure. Cold liquid stream 310 isintroduced into system 302 by upper concentrate pump 318 located at theupper end of riser 316. Stream 310 may if desired contain onlyconcentrate or brine 110, preferably is diluted by cold seawater 102that enters the exposed lower open end 320 of riser 316 beneath ringfloat 322. Doing so desirably decreases the salinity and increases thecold thermal energy potential of stream 310. Warm liquid stream 312 isobtained from shallow depth seawater near platform 304, and is pumpedinto system 302 by OTEC warm seawater pump 313. Warm liquid stream 312may be screened, filtered or otherwise pretreated (e.g., with a biocide)to reduce entrainment of marine life and biofouling. In anotherembodiment that may be available at select locations, warm liquid stream312 is obtained by capturing the output from an undersea thermal vent.

System 302 employs thermal energy extracted from the temperaturedifferential between cold liquid stream 310 and warm liquid stream 312to provide electrical power for operation of electrically-drivencomponents in the disclosed SRO apparatus. Following utilization of suchthermal energy, streams 310 and 312 may be separately discharged, but inthe embodiment shown in FIG. 3 preferably are mixed to form wide areadispersal stream 314. Doing so helps further reduce the salinity ofstream 314.

If desired, an open-cycle OTEC system may be used in place of aclosed-cycle OTEC system. In an open-cycle system, the warm seawater isconverted to vapor using a suitable evaporator. The vapor drives aturbine before being condensed to freshwater by the cold concentrate orbrine stream. The condensed freshwater may be combined with productwater from the ocean well to provide additional freshwater production.An open-cycle system avoids the need for a circulating working fluid andthe associated circulation pump. Also, it generally is not necessary toemploy screens, filtration or a biocide on the warm water side of thesystem, as the evaporation process normally is sufficient to preventbiofouling. A hybrid cycle OTEC system that combines features of bothopen and closed-cycle systems may also be employed. In general however,a closed-cycle OTEC system will be preferred for most applications.

FIG. 4 shows a schematic view of the disclosed closed-cycle OTEC system302. Cold stream 310 and warm stream 312 are respectively used by system302 to repeatedly liquefy (in condenser 402) and volatilize (inevaporator 404) a volatile circulating working fluid 406 such as ammoniaor a hydrofluorocarbon circulated by OTEC working fluid pump 408. Warmstream 312 and cold stream 310 preferably have a temperaturedifferential, as measured upon their arrival at system 302, of at least18° C., at least 20° C., at least 22° C., at least 24° C. or at least26° C. Based on current ocean temperature conditions, preferred suchtemperature differentials are generally found in offshore locationsbetween the Tropic of Cancer to the north of the equator and the Tropicof Capricorn to the south. Somewhat reduced temperature differentialsmay be found at numerically higher northern or southern latitudes, aswell as in the tropical latitude offshore regions lying just to the westof Africa and South America.

The differing thermal energy potentials of cold stream 310 and warmstream 312 enable working fluid 406 to repeatedly change from a vaporphase to a liquid phase and back to a vapor phase while circulatingthrough condenser 402, working fluid pump 408 and evaporator 404, withthe volume expansion caused by vaporization serving to drive turbine 410and its coupled electrical generator 412. A portion of the electricaloutput 416 from generator 412 may be used to drive pump 408. Anotherportion of electrical output 416 may be used to drive pumps 114 and 120using power respectively supplied via electrical cables 324 and 326shown in FIG. 3. Another portion of electrical output 416 may be used todrive other submerged pumps such as pumps 313 and 318. Any remainingelectrical output may be used for other purposes in connection withoperating, monitoring or maintaining the SRO apparatus, or usedexternally (e.g., onshore).

Referring again to FIG. 3, pumps 120 and 318 promote inflow and mixingof cold seawater 102 into the exposed lower open end 320 of riser 316and the formation of diluted cold concentrate or brine stream 310.Stream 310 has generally reduced salinity compared to concentrate orbrine 110 and thus presents a reduced potential hazard to marine life.Stream 310 also has substantially greater volume and substantiallygreater cold thermal energy potential (e.g., for the above-mentioned SROapparatus based on Dow FILMTEC membranes, about 11.5 times the volumeand about 11.5 times the cold thermal energy potential) than would beavailable if an SRO product water stream was instead employed to coolthe OTEC system. Accordingly, using a cold brine or concentrate stream(and especially using a low recovery ratio concentrate stream that isfurther diluted with cold seawater) for OTEC cooling can provide asignificant improvement in overall OTEC energy efficiency compared to anRO system using cold product water for OTEC cooling. Doing so can alsohelp reduce both CAPEX requirements and operating expenses. Inparticular, when the RO membranes are operated at deliberately lowerrecovery ratios than might be used in a shore-based system, theconcentrate volume can become very large relative to the product watervolume (e.g., up to 10-20 times the permeate volume at 5-10% recovery),thereby greatly facilitating the extraction of useful energy via OTECsystem 302. In preferred embodiments the disclosed Ocean Well+OTECsystem consequently can be optimized to provide both a 100% self-poweredseawater desalination system as well as surplus power for a variety ofuses. For example, system 302 may be sized to provide surplus electricalpower to operate warning beacons, monitoring equipment, telemetrydevices, ROVs and other electrical devices used in or with the disclosedSRO apparatus. In additional preferred embodiments, system 302 is sizedto provide further additional electrical power for use in other nearbySRO units or for onshore delivery via suitable cabling for private orpublic onshore use.

Combining OTEC power generation with the use of and discharge of dilutedconcentrate accomplishes particularly important goals, including (1)transport of concentrate or brine 110 to an OTEC system located at ornear the surface 124 moves the concentrate or brine 110 far away fromboth the disclosed SRO apparatus seawater intake and the ocean floor136, where a buildup of salinity would be detrimental to thedesalination process and the benthic environment, respectively; and (2)mixing cold seawater 102 and warm seawater 312 with concentrate or brine110 can dilute the concentrate or brine 110 to negligible levels ofelevated salinity, such that dispersal stream 314 may, depending on thejurisdiction, not be classified as “brine”. In any event, stream 314 maybe controlled to pose little or no environmental threat. If desired,dispersal stream 314 may also or instead be pumped from system 302 usingan airlift pump as discussed in the above-mentioned InternationalApplication WO 2018/148542 A1 (Bergstrom). Doing so can help oxygenatethe dispersal stream, thereby promoting increased oxygenation of nearbyseawater and a reduction in hypoxia.

As also depicted in FIG. 3, riser 316 preferably has a generally conicalshape, with a narrow lower end proximate the top of riser 122 and a wideupper end proximate the base of platform 304. This shape helps maintaina sizable diameter water flow path through riser 316 despite bending orother constriction in riser 316 that may be caused by lateral or othermovement of apparatus 100 and platform 304 relative to one another. Thedisclosed generally conical shape can also help reduce pipe frictionnear platform 304 (where there may be the most lateral motion) and canhelp reduce material stress distributions throughout riser 316 that mayresult from such motion. Riser 316 may be rigid or flexible and may bemade from a variety of materials including insulated or uninsulatedseawater-resistant fabric, textile, polymeric or composite material.Riser 316 preferably is both flexible and insulated and more preferablyis made from an insulated and imperviously-coated flexible textilelaminate. In one embodiment, riser 316 is also slidably mounted withrespect to riser 122, thereby compensating for vertical movement (e.g.,as may be caused by waves notwithstanding the retaining forces appliedby mooring lines 306) of platform 304.

In an additional embodiment (not shown in FIG. 3), system 300 may employone or more added devices to increase the warm-side stream 312 intaketemperature, such as a suitably large floating or otherwise permanentlyor temporarily-exposed solar absorbing heat exchanger or solar pondwhose color, texture, and geometry are tuned to absorb greater solarenergy than would be absorbed by seawater occupying the same area.

As an alternative to the embodiment shown in FIG. 3, OTEC unit 300 mayif desired be placed on a floating or fixed exposed platform located ator above surface 124. As shown for example in FIG. 5, sea level platform500 may be anchored as described above or otherwise maintained at sealevel. In some embodiments, the depth of platform 500 may be adjusted asneeded to account for changing wave heights, tides or projected sealevel changes. Moving or other replaceable parts on platform 500desirably will be located mainly at depths readily reachable by humandivers, e.g., at depths less than about 100 meters, less than about 60meters, less than about 50 meters or less than about 30 meters. One ormore auxiliary or alternative power sources on platform 500 such asunderwater (viz., tidal) turbines 502, other conventional turbines (notshown in FIG. 5), wave energy generator 504, solar panels 506 or windturbine generator 508 may be used to operate or help operate OTEC unit300, or may be used to independently operate submerged pumps and otherelectrical components in the disclosed SRO apparatus. Suitableunderwater turbines include those available from Nova Innovation.Suitable wave energy generators include those available from CalWavePower Technologies, Inc. Suitable solar panels include bothplatform-mounted and floating designs such as those available fromKyocera Corporation. Suitable wind turbine generators include thoseavailable from Principle Power, Inc.

Apparatus 100 may if desired be instead or in addition supplied withpower from a conventional platform-mounted, ship-borne or onshore powersource, for example an onshore power plant. Doing so may result ingreater carbon emissions than when using the power supply systemsdisclosed in FIG. 3 through FIG. 5, but may be desirable in instances inwhich the disclosed OTEC system and the disclosed auxiliary oralternative power sources on platform 500 are not able to providesufficient power or are not able to do so at all times or seasons whensuch power may be needed.

As also shown in FIG. 5, discharge conduit 510 may be used to transportconcentrate or brine exiting the disclosed OTEC system away from theOTEC system for release into the surrounding water. The overalldirection for such transport may be horizontal, upward, or (as shown inFIG. 5) downward with respect to the OTEC system. In a preferredembodiment, the effluent from conduit 510 is a reduced salinity blendedeffluent made by combining the disclosed cold concentrate or brinestream and warm water stream after these respective streams have passedthrough the OTEC system and their desired thermal energy potentials havebeen extracted. In a further preferred embodiment, a significant part(e.g., at least 20%, at least 30%, at least 40% or at least 50%) of theeffluent in conduit 510 exits the conduit via a series of side orifices512 arrayed along the terminal end of conduit 510, and the remainingeffluent exits via end orifice 514. In a further preferred embodiment,the effluent from conduit 510 is discharged into a water column at adepth at which the surrounding seawater density exceeds that of theeffluent (e.g., the blended effluent), thereby reducing atmosphericrelease of carbon dioxide compared to discharge at a lesser depth.

Referring to FIG. 6, a “water farm” containing an array of portableoffshore desalination systems (“pods”) 600 is shown in perspective view.Product water flows downwardly from the modules 600 through conduits 602and horizontally through pumps 604 to a centrally located hub 606, andis then pumped towards the surface through delivery conduit 608.Concentrate or brine is pumped upwardly through conduits 610 into oceancurrents for dispersal away from the pods 600 or for use in an OTECsystem like that discussed above. The conduits 610 may if desired bekept separate from one another, bundled together, or connected to asingle larger diameter conduit, and may if desired by equipped withhot-swap water connectors (not shown in FIG. 6) to facilitatedisconnection, maintenance or replacement of individual pods 600.

As depicted in FIG. 6, four pods 600 are employed. However, lesser orgreater numbers of pods can be used if desired, for example 2, 3, 5, 6,7, 8, 10, 20 or more pods. Using a plurality of connected pods providesredundancy and enables ready scaleup of the disclosed SRO apparatus tomeet initial or growing water needs. Operation and maintenance of thedisclosed apparatus can be facilitated by providing a plurality ofhot-swap water connectors (not shown in FIG. 6) between each conduit 602and its associated pump 604, or between each pump 604 and hub 606, or atboth the inlet and outlet ends of each pump 604. Scaleup of thedisclosed apparatus can be facilitated by providing one or moreadditional hot-swap water connectors (not shown in FIG. 6) on hub 606 orat another convenient location to enable connection of additional podsor water farm arrays to delivery conduit 608 at a later date. If forexample the individual pods 600 shown in FIG. 6 each have a 5 milliongallons per day product water capacity, and if five additional hot-swapconnectors are included in hub 606, then the FIG. 6 water farm couldprovide 20 million gallons of product water per day as initiallyinstalled, and up to five additional similarly-sized pods 600 could beadded in 5 million gallons per day increments to provide up to 45million total gallons of product water per day. In another embodiment, aplurality of such arrays may be installed near one another to providemultiple instances of the 20 million gallon per day array shown in FIG.6, thereby providing increased capacity, redundancy and multiplicity ofscale for the individual components. In yet another embodiment, the podsare not grouped together as depicted in FIG. 6, and instead are spacedapart across the seafloor, for example to accommodate topographicalchanges in the seafloor landscape, mooring line locations or othersubsea features.

FIG. 7A shows a perspective underside view of a polygonal array 700 ofthe disclosed submerged RO membrane modules 106 mounted beneath hood118, with prefilter system 104 being removed and with four of the twelvemodules 106 in the polygonal (viz., dodecagonal) array 700 beingnumbered in FIG. 7A (as modules 106A, 106B, 106K and 106L) and theremaining eight modules being unnumbered. Modules 106 have generallytapered module sides that converge towards centrally-located productwater (viz., permeate) collector 112 and product water collectionconduit 113. Modules 106 are in fluid communication with collector 112and conduit 113 via hot-swap product water valves 706 mounted oncollector 112, with three of the twelve valves connected to array 700and conduit 113 being numbered in FIG. 7A (as valves 706A, 706B and706K) and the remaining nine valves being unnumbered. Converging sides(two of which are numbered as side 702C and side 704C) on each module106 assist in underwater docking and reattachment of a detached module106 to a hot-swap product water valve 706. Central rails on modules 106(one of which is numbered as rail 703C) provide further support for themodules 106. When the modules 106 are in operation, product water flowsdownwardly through permeate collector 112 and permeate conduit 113 andis carried away by a pump such as pump 114 in FIG. 2.

The disclosed hot-swap product water valves and associated componentsmay utilize a variety of designs, including so-called “hot stab” checkvalves and receptacles like those used in undersea oil and gas equipmentfor handling hydraulic fluids. Suitable such valves and receptacles areavailable from a variety of suppliers including Blue Logic, FES SubseaEngineering Products, James Fisher Offshore, Oceaneering and TotalMarine Technology and Unitech. By way of example, the M5 ROV FlyableFull Bore Connector from Oceaneering represents one useful such hot stabvalve and receptacle combination. Because hot stab devices are typicallydesigned for use in the undersea oil and gas industries and musttolerate the handling of hydrogen sulfide and other corrosiveingredients at significant pressures, they can be derated and theirdesigns can be simplified and made less expensive when used to handlethe noncorrosive or less corrosive fluids and much lower pressurespresent in the disclosed SRO apparatus.

FIG. 7B shows a perspective underside view of the disclosed SROapparatus of FIG. 7A without array 700 and its modules 106. Hood 118includes a supporting framework formed by inclined struts 708, crossbars710A, 710B and 710C, lower circumferential rim supports 712, lowerradial rails 714, lower inner anchoring ring 716 and an upper inneranchoring ring located (but not shown in FIG. 7B) at the junction ofhood 118 and riser 122. The disclosed framework preferably alsoreceives, captures and supports the disclosed modules and array. Eachcircumferential rim support 712 includes a slotted receiving aperture718 that captures hangers atop each module 106, and which are discussedin more detail below. The disclosed framework supports an overlying,impervious protective hood cover 128 that may for example be made froman insulated or uninsulated seawater-resistant textile, plastic or metalcovering material. In a preferred embodiment, the inner side 720 of hoodcover 128 is made for example from a buoyancy-imparting material such asan engineered syntactic foam. Hood cover 128 preferably provides aprotective cover that helps maintain a slight pressure differential(e.g., up to 50 psi or thereabouts) between the internal concentrate orbrine collected by the hood and the external environment, whileisolating the interior of the hood from penetrants. Indentations 722 andproduct water valve couplings 724 may be seen near the top of conduit113, just below ring 716. Rails 714, apertures 718 and indentations 722help guide, support and locate modules 106 when array 700 is installed,and couplings 724 assist in the hot-swap attachment and detachment ofmodules 106. If desired, corrosion resistant magnets or electromagnetsmay be used to guide, retain or both guide and retain modules 106 inplace within array 700 and apparatus 100.

FIG. 7C shows a perspective underside view of the disclosed SROapparatus of FIG. 7A with prefilters 104 installed. In the embodimentshown in FIG. 7C, each prefilter 104 has a generally triangular shape,and is periodically swept clean of debris by oscillating brush arms 726mounted on pivot points 728 near lower mounting ring 730. Brush arms 726repeatedly (e.g., intermittently, periodically or continuously, andbased on predetermined times, signals from one or more sensors, or anexternally-supplied control signal) sweep across the inlet face of eachprefilter 104 toward central struts 732 and then stop, return to thepositions shown in FIG. 7C, or initiate another sweep movement.Crossbars 734 help reinforce and support each prefilter 104 and canserve as guide rails supporting brush arms 726. In another embodiment,the assembled prefilters may have curved surfaces, a generally conicaloverall shape, and brush arms configured to sweep over or to revolvearound the prefilters.

FIG. 8A and FIG. 8B are perspective topside views of a module 106showing a plurality of RO membrane cartridges 802 suspended and sealedin apertures 804 in perforated, generally triangular divider plate 806.As depicted, cartridges 802 are generally cylindrical but may have othershapes if desired. As also depicted, module 106 contains 142 cartridges,but other greater or lesser numbers of cartridges may be employed ineach module as desired, for example at least 40, at least 50, at least60, at least 70, at least 80, at least 90 or at least 100 cartridges,and up to 200, up to 190, up to 180, up to 170, up to 160, up to 150 orup to 140 cartridges. Also, as depicted all the cartridges are generallyparallel and in a single layer occupying a single plane, as doing sopromotes efficient flow through the disclosed apparatus. However, ifdesired the cartridges in a module need not be generally parallel to oneanother, and also if desired multiple layers of cartridges could beemployed in a module.

Using 140 of the above-mentioned Hydranautics cartridges in each module,the disclosed SRO apparatus may produce about 5 million gallons per dayfrom a twelve such modules operated at a 5% recovery rate. Other ROmembrane suppliers whose cartridges may be used will be apparent topersons having ordinary skill in the desalination art, and includeAquatech International, Axeon Water Technologies, DuPont Water Solutions(makers of the above-mentioned DOW FILMTEC cartridges), Evoqua WaterTechnologies, GE Water and Process Technologies, Koch Membrane Systems,Inc. and LG Chem. Customized cartridges having altered features (forexample, wider gaps between layers, modified spacers, a looser membraneroll, a modified housing or modified ends) may be employed if desired.

As depicted, the cartridges 802 are substantially vertically alignedwhen module 106 is installed in array 700 and in use, with theconcentrate or brine end outlets 804 in each cartridge 802 facingupwardly towards hood 118 and with the product water outlets (discussedbelow in connection with FIG. 8C) facing downwardly. However, otherorientations and accompanying flow directions may be employed, forexample with the outlets 804 facing downwardly, horizontally orobliquely, and with the product water outlets facing upwardly,horizontally or obliquely.

In certain embodiments, the cartridges 802 are mounted in the disclosedmodules 106 by adhesively bonding and sealing the cartridges in holes inperforated divider plate 806. In the embodiment depicted in FIG. 8A andFIG. 8B, the adhesive bond may be at the upper end of the cartridges 802near the concentrate or brine outlets 804. However, as depicted in FIG.8G, more than one divider plate 806 may be employed, and the dividerplate(s) and the adhesive 840 that bonds and seals the cartridges 802into the perforations 842 in divider plate 806 may be located at eitheror both ends or anywhere along the length of the cartridges 802. FIG. 8Galso illustrates the use of adhesive 840 to bond manifold 828 topermeate outlet 827 as discussed above. To provide greater buoyancy (anddesirably neutral buoyancy at the intended operating depth) and as shownin FIG. 8F, the spaces between cartridges 802 are desirably filled or atleast partially filled with a suitable buoyant medium 850 having adensity less than that of seawater. A preferred such medium isengineered syntactic foam, available from suppliers including EngineeredSyntactic Systems. Medium 850 may be provided in the form of shapedblocks that may be inserted into the modules 106 before or after theinstallation of the cartridges 802, as a premolded perforated slab thatis inserted into the modules 106 before installation of the cartridges802, or as an in-situ cured material that may be placed in spacesbetween the cartridges (e.g., by spraying or other form of injection)after the cartridges have been added to the modules 106. In anembodiment, beads of adhesive 840 may be omitted and medium 850 caninstead serve as the adhesive that bonds and seals the cartridges 802into the perforations 842 in divider plate 806. Use of medium 850 cansignificantly assist the underwater removal and installation of themodules 106, by reducing the cantilever effect of the mass of eachmodule 106 as it is being removed from or flown into position in thedisclosed array, and especially when such removal and installation areconducted using an ROV that grips the outer edge of a module 106.

Perforated divider plate 806 may have a variety of shapes, for example agenerally polygonal perimeter such as generally triangular perimeter ora generally trapezoidal perimeter. Plate 806 and the remainingcomponents in module 106 that support and envelop (viz., provide a framefor) the cartridges 802 may be made from a variety of materials,including corrosion-resistant metals such as stainless steel ortitanium, fiber-reinforced polymers or filled composites. Preferably amixture of such materials is employed, with lower density componentsbeing used in appropriate locations to reduce the overall module weight,and higher strength or higher durability materials being used in otherappropriate locations within the module where such strength ordurability may be required. Divider plate 806 and the remainingcomponents in module 106 may if desired be surface-treated to resistbiofouling, and may also be surface-treated to increase the associatedsurface area in locations that may require improved adhesion by adhesive840.

A variety of adhesives may be used to bond and seal the cartridges inthe modules. Exemplary adhesives include the above-mentioned engineeredsyntactic foams, as well as epoxy, polyurethane, polyester, acrylic,silicone and fluorinated resins. In one preferred embodiment, theadhesive is substantially free or completely free of bisphenol A,bisphenol F and their diglycidyl ethers. In another preferredembodiment, there are no gaskets, O-rings or other preformed sealsbetween the cartridges and the divider plate and the adhesive isprimarily or exclusively relied upon to hold the cartridges 802 in themodules 106. Suitable adhesives will include those classified as marineadhesives or sealants suitable for use below the waterline, and areavailable from a variety of suppliers including Dow Chemical Company,Loctite, Sika and 3M. In a further preferred embodiment, the cartridgesare not encased in a pressure vessel.

When a module 106 is removed from the disclosed array for replacement ofone or more of the cartridges 802, it may in some instances be desirableto remove and replace only certain of the cartridges, and in otherinstances it will be most economical to remove and replace all of them.Removal typically will require debonding the affected adhesive joints sothat the associated cartridges 802 may be extracted from divider plate806. Depending on the chosen adhesive, debonding may be performed usinga variety of techniques. Exemplary techniques include chemical debonding(e.g., using solvents, hydrolysis, or other measures), cryogenicdebonding (e.g., using liquid nitrogen), electrical debonding (e.g.,using current from an arc welder or other power supply to heat aconductive filler in the adhesive) or thermal debonding (e.g., using aflame or other heat source) to dissolve, fracture, soften melt, orotherwise weaken or degrade the adhesive or its bond to the cartridgesand divider plate. Once the adhesive bond has been sufficiently weakenedor degraded, the cartridges may be pushed, pulled, twisted or acombination thereof to remove them from the module.

Use of the disclosed adhesive provides a number of advantages. Waterseparation membrane cartridges are normally sealed to other componentsin a water desalination apparatus using gaskets or O-rings. Gaskets andO-rings represent a potential array leakage point, especially if thegasket or O-ring undergoes significant compression set upon exposure tocold underwater temperatures. Adhesively bonding the membrane cartridgesto a perforated divider plate eliminates this potential leakage pointwhile meanwhile increasing the beam strength and rigidity of theassembled module.

Referring again to FIG. 7B and FIG. 8A, the lower edges of hood 118preferably overlap with, have a gasketed connection to, or are otherwisesealingly engaged with the upper edges of the modules 106, therebyisolating the collected concentrate or brine from the salinated watersurrounding module 100. Divider plate 806 is desirably fastened andsealed about its periphery to converging side plates 702 and 704, outerend plate 812 and inner end plate 814, thereby further isolating thecollected concentrate or brine from the surrounding salinated water.

As depicted in FIG. 8A through FIG. 8C, suspending hooks 816 and 818 arefastened atop and near the outer edge of module 106, and point towardthe inner edge of module 106. Hooks 816 and 818 mate with slottedreceiving aperture 718 shown in FIG. 7B and help support and guidemodule 106 into a proper position when module 106 is pushed into anavailable open space in array 700. Guide rails 820 and 822 engage radialrails 714 beneath hood 118 during insertion of a module 106 into array700. Generally wedge-shaped projecting tang 824 also helps to guide andproperly affix module 106 into place in array 700, and helps ensureproper connection of hot-swappable product water valve 706 to permeatecollector 112. Upon the completion of such connection, hot-swap valve706 opens to permit the flow of product water into permeate collector112 and permeate conduit 113.

FIG. 8C, FIG. 8D and FIG. 8E are perspective underside views of a module106. The circular salinated water inlets 826 at the lower end of eachcartridge 802 permit the entry of salinated water into the cartridges802. Desalinated product water exits a typically centrally-locatedoutlet 827 in each cartridge 802 via cartridge manifolds 828, branchmanifolds 830A through 830I (for the nine depicted rows in the disclosedcartridge array that contain two or more cartridges each side of thearray centerline) and a pair of radially-extending product watercollection manifolds 832 located each side of the array centerline.Three single cartridges 802 located on each side of the array centerlinenear inner end plate 814 are directly connected by individual cartridgemanifolds 828 to product water collection manifolds 832.

The prefilter screens 104 shown in FIG. 7C, sides 702, 704, 812 and 814and the underside of perforated divider plate 806 cooperate to isolatethe filtered water passing through prefilter screens 104 from thesalinated water surrounding module 100 and ensure that the interiorportion of module 106 surrounding the cartridges 802 will contain onlyfiltered water that has passed through a screen 104.

In the embodiment depicted in FIG. 7A through FIG. 8E, the modules 106have an approximately wedge-shaped or trapezoidal-shaped cross-section,with sides 702 and 704 that taper or converge towards permeate collector112 and the vertical central axis of the disclosed SRO apparatus. Thedisclosed modules may be any desired size, and in a preferred embodimentmay for example be about 5 to 8 meters long by about 4 to 5 m wide byabout 0.5 to 1.5 m thick, and have shapes and dimensions that facilitateefficient packing of the modules in standard shipping containers asdiscussed in more detail below. The disclosed modules preferably haveneutral buoyancy at the intended operating depth.

In the embodiment depicted in FIG. 9, twelve modules are shown, and intheir assembled form the depicted modules provide an array with adodecagonal perimeter in plan view. Other module shapes, numbers ofmodules (e.g., 3, 4, 5, 6, 7, 8, 9, 10, 11, 13, 14, 15 or 16) and arrayshapes (e.g., triangular, square, pentagonal, hexagonal, hexadecagonal,and other polygons made from the numbers of modules mentioned above, aswell as circular or other curved shapes) can be employed if desired.

As illustrated in FIG. 9, maintenance of an array 900 containing adefective, outdated or otherwise ineffective module may be performed bywithdrawing such module from the array, thereby leaving a gap formerlyoccupied by the withdrawn module, and replacing the withdrawn modulewith a new or rebuilt module 906L. Because such replacement will becarried out while the SRO apparatus is submerged, it is desirable thatboth the module removal and module replacement procedures proceedquickly and efficiently, with minimal disruption in product water outputdespite potential adverse conditions such as low visibility, underwatercurrents or difficulties in operating an ROV or other devices that mightbe used to carry out or assist in module replacement. Module maintenancemay for example be scheduled or performed based on signals from one ormore sensors that monitor the flow rate or salinity of product water,concentrate or brine flowing through the apparatus, or the flow of waterinto the apparatus. Such sensors may monitor individual cartridges,individual modules or an entire array. Module maintenance may inaddition or instead be based on a preset or adjustable schedule,predictive algorithms, the availability of improved RO membranes orcartridges, and other measures that will be apparent to persons havingordinary skill in the desalination art upon reading this disclosure.

Removal of defective or ineffective modules can be facilitated whilecontinuing to operate the remainder of the disclosed apparatus duringmodule removal, and relying on the portion of the disclosed hot-swapvalve 706 that remains connected to permeate collector 112 to close andseal off permeate collector 112 from the surrounding salinated water.Such valve closure may be initiated in a variety of ways, including inresponse to a suitable electrical command, mechanical switch, or inresponse to the outward motion of a module 106 away from permeatecollector 112 and separation components in hot-swap valve 706. Hot-swapvalve 706 accordingly desirably prevents the entry of salinated waterinto permeate collector 112 during module replacement. The portion ofhot-swap valve 706 remaining on the removed module 106 may optionallyalso be closed upon removal in order to prevent entry of desalinatedwater into the product water outlet side of the removed module 106.However, doing so generally will not be needed, as the removed module106 will normally be brought to the surface and flushed with fresh wateras a part of a repair or rebuilding procedure.

Removal of a module 106 may cause unfiltered salinated water to enterthe otherwise normally isolated chamber between the prefilters 104 andthe modules 106 in the disclosed array 700. Typically however suchunfiltered water entry would take place for a relatively brief timeperiod, until such time as a replacement module or temporary blankingplate can be inserted into the array, and consequently will be unlikelyto introduce significant detrimental quantities of debris or other solidmatter into such chamber.

During insertion of replacement module 906L, converging sides 908 and910 and hangers 916 and 918 assist in underwater docking and attachmentof module 906L to array 900 by helping to align and guide module 906Linto proper orientation and location in the three-dimensional volumebetween adjacent modules 906A and 906K, and by helping to align andguide hot-swap valve body 920 into proper alignment and engagement withthe portion of valve body 706 that remains attached to permeatecollector 112. Hangers 916 and 918 preferably have inwardly-pointingtapered ends (viz., ends that point towards the longitudinal centralaxis of the disclosed array and have a wedge-shaped profile in planview, side view or both plan and side views). Such tapered ends willsignificantly assist in docking module 906L into the disclosed SROapparatus. During insertion of module 906L into array 900, hangers 916and 918 enter the slotted receiving aperture 718 shown in FIG. 7B, andrails like the rails 820 and 822 shown in FIG. 8A and FIG. 8B engagewith the radial rails 714 shown in FIG. 7B. Upon reattachment ofreplacement module 906, the hot-swap valve formed by valve bodies 706and 920 can then open to permit resumption of product water collectionfrom the affected portion of the disclosed array. Such valve opening maybe initiated in a variety of ways, including in response to a suitableelectrical command, mechanical switch, physical manipulation by an ROV,or in response to the inward motion of module 906L and joinder of valvebodies 706 and 920. In this fashion, removal and inspection orreplacement of individual modules 106 can be accomplished without havingto shut down the disclosed SRO apparatus, thereby enabling continuedproduction of product water and concentrate or brine from the remainingundisturbed modules 106.

In addition to the disclosed tapered sides, rails and hangers, thedisclosed module reattachment procedure may be assisted by employingother guidance features or devices. Exemplary such other features ordevices will be apparent to persons having ordinary skill in thedesalination art upon reading this disclosure, and includeappropriately-shaped (e.g., conical or tapered) mating or receivingsurfaces, snubbers, guiderails or magnets on the sidewalls of thereplacement module or adjacent modules, the upper or lower surfaces ofthe replacement module, adjacent portions of the framework receiving thereplacement modules, or the hot-swap valve bodies 920. Such otherguidance features or devices may for example contact the replacementmodule or adjacent modules during any or all of the start, middle, orend of the disclosed replacement procedure. If desired, one or moregaskets may also be employed on the modules 106, hood 118 or assembly ofprefilters 104 to assist in sealing gaps between the modules 106 and theremainder of the disclosed SRO apparatus, and in some embodiments suchgaskets may provide guidance features to assist during module insertion.

Maintenance requirements may be further simplified by employing suitablehot-swap connectors for other fluid or electrical connections betweenpotentially replaceable components and the disclosed apparatus, andespecially for components that may be subject to wear or prematurefailure. Such components may include wearable moving components such aspumps, valves, intake screen wipers and the like; electrical componentssuch as motors, sensors, transformers, variable frequency drives (VFDs),and the like; and fouling-prone components such as intake screens.Desirably such connectors and the associated component(s) can bedetached from and reinserted into the disclosed apparatus using asuitable ROV.

Shipment and storage of dry (viz., non-submerged) may be made morespace-efficient by placing two generally triangular modules (or twogenerally triangular portions of a module) alongside one another andfacing in opposite directions to provide a generally rectangular,dense-packed shipment or storage package. As shown in FIG. 10A, this mayfor example be performed by omitting parts near the major centerline ofa module 106 (such as the hot-swap product water valve 706) that cannotconveniently be split in half, and splitting the remainder of the module106 at or near its major centerline to provide modular halves 1000, 1002and 1004. When modular halves 1002 and 1004 are placed alongside oneanother and facing in opposite directions, they form a generallyrectangular pair 1006. As shown in FIG. 10B and FIG. 10C, a plurality(e.g., four) half-module pairs 1006 may be stacked together to provide acompact, readily transportable shipment 1008 containing eighthalf-module pairs 1006, which once the half-modules are combined withthe omitted parts and reunited with one another, will correspond to fourfull modules 106. One or more such half-module pairs preferably fit inand substantially fill (e.g., to occupy 80% or more of the availablefloor space of and preferably the available interior volume of) astandard ocean shipping container for subsequent shipping and transport,or may be transported in enclosed or unenclosed fashion on a flatbed orother trailer 1010 by truck cab 1012. Representative standard shippingcontainers have typical widths of about 2.4 m (8 ft.), heights of about2.6 m (8.5 ft.) and nominal lengths of about 3 m (10 ft.), 6.1 m (20ft.) or 12.2 m (40 ft.).

Referring now to FIG. 11 and FIG. 12, the above-mentioned prefilterscreens provide prefiltration for incoming salinated water that canreduce maintenance requirements in the disclosed SRO apparatus. Thedisclosed SRO apparatus typically will be operated at a significantdepth, far from human hands, and it consequently will be desirable toemploy stringent measures to avoid RO membrane fouling. Some prior SROdesalination proposals appear to assume that deep seawater issufficiently clean to permit desalination without prefiltration. In someembodiments (e.g., in appropriately clean waters or when using membranesthat are less susceptible to fouling), pretreatment may be omittedentirely. However, although seawater at depths of for example 700 metersis typically vastly cleaner than surface water, such seawater maynonetheless contain sufficient contaminants to require periodicreplacement of the RO membranes to prevent or overcome clogging orfouling. The disclosed prefilter provides a one-stage and preferably atwo-stage prefiltration treatment that can prolong RO membrane lifetime.By performing pretreatment at depth rather than near the shoreline, theoverall likelihood of lethal marine life entrainment is reduced. Theprefilter can be periodically or continuously back flushed or otherwisepurged using air bubbles or an air/water stream to further prolong theprefilter lifetime while meanwhile further minimizing or avoiding harmto marine life. If desired, chlorine or ozone may be introduced near theprefilter, in order to disinfect the prefilter and discouragebiofouling. By using the relatively cleaner waters available at thedisclosed preferred depths and a prefilter cleaning procedure, thedisclosed SRO apparatus can significantly reduce the need for prefilterreplacement or RO membrane replacement that might otherwise be requiredfor shore-based or shallow depth RO, while meanwhile reducingaccompanying capital, operating, real estate and energy requirements.

FIG. 11 and FIG. 12 respectively show prefilter outlet side andprefilter inlet side schematic views of a system for cleaning prefilterscreens that are similar but not identical to the prefilter screens 104shown in FIG. 7C. Referring first to FIG. 11, prefilter 1100 includes aplurality (ten, as depicted in FIG. 11) of screens 1102 which aremounted on and surrounded by perimeter flange 1104, crosspieces 1106 andcentral flange 1108. Pivot bearings 1110 support movable cleaning arms1112. When suitable electrical or hydraulic energy is supplied to amotor (not shown in FIG. 11 or FIG. 12) connected to the arms 1112, thearms 1112 sweep back and forth in an arc across the output face of thescreens 1102. The arms 1112 may be moved in continuous fashion or atdiscontinuous, periodic, random or other intervals. Pressurized water orair supplied through the arms 1112 passes through water or air backflushjets 1114 that dislodge debris from the screens 1102.

Referring next to FIG. 12, arms 1212 are moved by the above-mentionedmotor back and forth over the surfaces of screen 1102. The arms 1212include suction ports 1214 and mechanical brushes 1216 that removedebris present on prefilter screen 1102. The suction ports 1214 alsoremove debris dislodged from the screens 1102 by the backflushing actionof the jets 1114. In one embodiment, the brushes 1216 are moveable(e.g., rotatable) with respect to their respective arms 1212 as the armstraverse back and forth across the screens 1102, in order better todislodge such debris.

The disclosed SRO desalination apparatus may be operated in a variety oflocations. In one preferred embodiment, the apparatus is deployed in anocean trench or dropoff (for example, the Monterey Submarine Canyon,Puerto Rico Trench, Ryukyu Trench, waters surrounding the HawaiianIslands, and other accessible deep sea sites that will be familiar topersons having ordinary skill in the desalination art), near a populatedarea in need of desalinated water. The SRO inlet surfaces need not beplaced at trench floor depth, and may instead be positioned along thetrench wall at a depth sufficient to enable the use of hydrostaticpressure to drive seawater through the osmotic membranes.

Operation at appropriate depths can greatly reduce or eliminate thelikelihood of algal bloom contamination, which can cause conventionalshore-based plants with shallow water intakes to shut down in order toavoid toxins and clogging. Operation at such appropriate depths can alsominimize or eliminate the loss of marine life, as most marine organismsare found within the photic zone (depending upon water clarity,corresponding to depths up to about 200 m) and thus at deeper depthswill not be drawn into the SRO apparatus intake or against a prefilterscreen.

The cold feedwater (e.g., 5-10° C. water) typically encountered at theabove-mentioned recommended SRO operating depths can provide severaluseful advantages. For example, the feedwater is relatively free fromcritical organic and inorganic contaminants. It carries very littleorganic matter or chlorophyll and thus contains little bacteria, whilestill retaining valuable nutrients from the ionic minerals and traceelements present at the disclosed pressures and depths. A furtheradvantage arises in connection with boron removal, which is importantfor irrigation water and health purposes. Boron is present in seawater,and at conventional RO operating temperatures such as are used inonshore RO units, enough boron may pass through the RO membrane toinhibit the growth of plants. Boron removal to agricultural standards of0.5 mg/liter in a conventional RO facility may require double treatmentof the water using a second RO pass, thus increasing capital andoperating costs. Boron removal by reverse osmosis is however highlytemperature-dependent, with lower amounts of boron and its salts passingthrough the membranes at colder temperatures. For example, boratepassage may be reduced by several percentage points for every reductionof 10° C. in feedwater temperature. Placement of the disclosed SROdevice in cold deep water consequently may help produce higher-qualitydesalinated water by improving the removal of boron and its salts whilesaving the energy, capital, and maintenance costs required for a doubletreatment system. Cold feedwater can also result in less overall saltpassage through the membrane, allowing for remineralization of theproduct water for taste reasons while maintaining a low level of TDS tomeet regulatory requirements. In addition, the use of cold feedwater cannearly eliminate the scaling of membranes by mineral deposition, asmeasured by the Langelier Index. Membrane scaling can be a problem withshore-based, shallow-intake RO units, and reduces system efficiency andlifetime. In the disclosed SRO apparatus, scaling is minimized becauseCO₂ will tend to be in equilibrium at the 5-10° C. temperatures at whichthe RO membranes may be operating. This can eliminate the need for theanti-scaling chemicals that often are employed in shore-based RO units.Biofilm growth, another form of membrane fouling, is alsotemperature-dependent, with more biofilm forming at warmer temperatures,and less at the low-temperature preferred operating environment of thedisclosed SRO apparatus. Biological activity and hence biologicalfouling are thus reduced due to the use of water from a region havinglow light, low oxygen, and cold water temperatures.

The disclosed SRO apparatus can produce significantly lowerconcentrations of salt in the brine stream than will be the case forconventional RO, as the elimination of the requirement for pressurevessels permits the RO membranes to be arrayed in parallel rather thanthe typical seawater desalination industry practice of 5-7 membranes ina serial arrangement. A parallel array eliminates a common failure pointin conventional RO systems, namely the o-ring interconnections betweenmembranes. A parallel arrangement may also permit higher product waterproduction per membrane. In addition, a parallel membrane arrangementcreates much less salty concentrate or brine than a train of singlemembranes operating in series, and the salinity of such concentrate orbrine can easily be adjusted by altering the membrane recovery ratio.The ability of the disclosed SRO apparatus to achieve low brine salinityis beneficial to sea life and allows easier dilution of the concentrateor brine. For example, when supplied with Southern California seawatercontaining about 34,250 ppm ambient TDS and operated at a 5% recoveryratio, the disclosed apparatus may provide concentrate containing onlyabout 36,049 TDS versus the near-doubling in discharge stream salinitythat may arise using conventional serially-configured onshore RO. A36,049 ppm TDS discharge stream would be less than 1800 ppm aboveambient, and thus well within the current brine discharge limit of 2,000ppm above ambient TDS for California waters.

A principal benefit of the overall disclosed SRO apparatus is itssignificantly reduced energy requirements. The artificial pressurizationof process water, the largest source of energy use in conventional ROdesalination, can be reduced or eliminated. The energy consumption andassociated greenhouse gas production to produce desalinated water usingthe disclosed SRO apparatus may consequently be significantly reduced.The associated capital expenditures and operating expenditures can alsobe significantly reduced, especially in comparison with those requiredfor onshore RO desalination. These and other advantages of the disclosedSRO apparatus thus may include one or more of:

-   -   Greatly reduced power consumption.    -   Reduced greenhouse gas emissions to desalinate a given quantity        of water.    -   Elimination of the artificial high-pressure environment used in        conventional RO and the accompanying pressure vessels, high        pressure piping, and fittings.    -   Reduced operation and maintenance requirements through        elimination of parts, and especially the reduction of        highly-pressurized connections.    -   Fewer precision parts requiring expensive alloys and other        exotic materials resistant to seawater corrosion.    -   Reduced or eliminated pretreatment equipment and its associated        operating capital and labor.    -   Reduced localized brine emission.    -   Parallel rather than series membrane configurations with even        lower-salinity brine discharge.    -   Pipelines to shore that are over 50% smaller in diameter, as        only product water is sent onshore, subject however to increased        required length.    -   Reduced boron content in desalinated water, making it suitable        for agriculture without further treatment.    -   Reduced bacterial content and bacterial fouling due to the use        of deep-sea intake water that is relatively free of undesirable        organic or inorganic contaminants.    -   Reduced susceptibility to desalination disruption caused by        algal blooms.    -   Reduced visibility or invisibility from shore.    -   Reduced onshore noise pollution.    -   Reduced susceptibility to destruction due to adverse weather        events, fires, terrorism or volcanic eruptions.    -   Reductions by as much as 90% in required onshore real estate.    -   Suitability for deployment as an “Ocean Well” that can provide a        sustained freshwater supply without aquifer depletion.

Having thus described preferred embodiments of the present invention,those of skill in the art will readily appreciate that the teachingsfound herein may be applied to yet other embodiments within the scope ofthe claims hereto attached. The complete disclosure of all patents,patent documents, and publications are incorporated herein by referenceas if individually incorporated.

1. A submersible water desalination apparatus comprising an array ofhot-swappable water separation membrane modules arrayed around andgenerally radially extending from a product water collection conduit; aplurality of hot-swap product water valves connected to the conduit anddetachably connected to generally radially-extending product watercollection manifolds in fluid communication with a plurality of waterseparation membrane cartridges within the modules; the modules having incross-section generally tapered module sides that converge towards theconduit and assist in underwater docking and attachment of a replacementmodule to a hot-swap product water valve.
 2. An apparatus according toclaim 1, wherein the water separation membranes are reverse osmosismembranes.
 3. An apparatus according to claim 1, wherein the array hasan approximately polygonal perimeter in plan view.
 4. An apparatusaccording to claim 1, wherein the array has an approximately pentagonalto hexadecagonal perimeter in plan view.
 5. An apparatus according toclaim 1, wherein the array has an approximately dodecagonal perimeter inplan view.
 6. An apparatus according to claim 1, wherein the moduleshave the same shapes and sizes and are interchangeable with one another.7. An apparatus according to claim 1, wherein the modules are generallywedge-shaped in plan view.
 8. An apparatus according to claim 1, whereinthe modules have a generally trapezoidal perimeter in plan view.
 9. Anapparatus according to claim 1, wherein each module comprises asupporting frame or housing made from corrosion resistant metal andsurrounding the water separation membranes in such module.
 10. Anapparatus according to claim 1, wherein each module comprises asupporting frame or housing made from fiber-reinforced polymer or othercomposite material and surrounding the water separation membranes insuch module.
 11. An apparatus according to claim 1, wherein the waterseparation membrane cartridges have generally cylindrical shapes.
 12. Anapparatus according to claim 1, wherein the product water collectionconduit represents or is proximate to and generally aligned with avertical central axis of the array, and the water separation membranecartridges are generally parallel to the product water collectionconduit.
 13. An apparatus according to claim 1, wherein the modules andarray are not encased in a pressure-resistant outer vessel.
 14. Anapparatus according to claim 1, wherein the apparatus further comprisesa framework that is detachably connected to and receives, captures andsupports the modules, and a hood that captures concentrate or brine fromthe water separation membrane modules.
 15. An apparatus according toclaim 1, wherein the apparatus further comprises one or more guidancefeatures or devices comprising hangers, hooks, snubbers, guiderails ormagnets that attract, contact or repel a replacement module or anadjacent module during underwater docking and attachment of areplacement module to a hot-swap product water valve.
 16. An apparatusaccording to claim 1, wherein the modules have shapes and dimensionsthat enable a pair of modules or portions of modules to be placedalongside one another facing in opposite directions to provide one ormore generally rectangular module pairs that will fit in andsubstantially fill the floor space of a standard ocean shippingcontainer.
 17. An apparatus according to claim 1, wherein the waterseparation membrane cartridges produce a volume of desalinated productwater and a volume of concentrate, and the product water volume issufficiently less than the concentrate volume so that the concentratedoes not contain sufficient salinity to qualify as brine in thejurisdiction where such concentrate is produced.
 18. An apparatusaccording to claim 1, further comprising one or more sensors thatmonitor the flow rate or salinity of desalinated water, concentrate orbrine flowing through the apparatus.
 19. An apparatus according to claim1, further comprising a Remotely Operated Vehicle (ROV) for modulereplacement.
 20. A method for maintaining a submerged water desalinationapparatus, the method comprising the steps of: a. undocking from asubmerged water desalination apparatus a first hot-swappable waterseparation membrane module that is a member of a plurality of suchmodules arrayed around and generally radially extending from a productwater collection conduit and in fluid communication with the conduit viahot-swap product water valves connected to the conduit and detachablyconnected to generally radially-extending product water collectionmanifolds in fluid communication with a plurality of water separationmembrane cartridges within the modules, the modules having incross-section generally tapered module sides that converge towards theconduit and assist in underwater docking and reattachment of detachedmodules to a hot-swap product water valve; and b. docking a similarlysized and shaped replacement module and attaching it to the array whileunderwater by guiding the converging tapered sides of the replacementmodule and a detached product water collection manifold on such moduletowards a detached hot-swap product water valve connection on theproduct water collection conduit.